1. Field of the Invention
The present invention relates generally to a fuel injection pump and, more particularly, to a fuel injection pump having a mechanism for the adjustment of fuel injection timing.
2. Prior Art
Such a type fuel injection pump is known in the art, as disclosed in, for instance, Japanese Utility Model Kokai-Publication No. 57 (1982)-142167. According to the fuel injection pump disclosed therein, a control sleeve is slidably fitted over a plunger to control the injection of fuel by varying the relative positions of the control sleeve and the plunger. FIGS. 5 and 6 show a conventional example of such a fuel injection pump.
Referring to FIGS. 5 and 6, a plunger 8 is rotatable and reciprocatable within a plunger barrel 3. A control sleeve 24 is slidably fitted over the plunger 8. The plunger 8 is provided with a longitudinal hole 31 which is open at one end in a fuel-pressurizing chamber 9 located thereabove, an inclined groove 40 on the outer face thereof, which is in communication with the longitudinal hole 31, a longitudinal groove 41 which is formed at one end of the inclined groove 40 and extends along the axial direction of the plunger 8, and a lateral hole 42 which is located at the lower end of the longitudinal groove 41 and is open to the longitudinal hole 31. On the other hand, the control sleeve 24 is moved vertically by the rotation of a control rod 43 engaging therewith, and is provided therein with a cut-off port 36. In such an arrangement as mentioned above, when the plunger 8 is rotated by means of a mechanism for regulating the amount of fuel to be injected (not shown), there is a change in the distance (effective stroke from the closing of the lateral hole 42 in the plunger 8 by the control sleeve 24 to the alignment of the inclined groove 40 in the plunger 8 with the cut-off port 36 in the control sleeve 24) whereby the amount of fuel to be injected is adjusted. When the control sleeve 24 is vertically moved by the control rod 43, on the other hand, there is a change in the distance (prestroke) from the initiation of ascent of the plunger 8 to the closing of the lateral hole 42 in the plunger 8 by the control sleeve 24, whereby the timing of fuel to be injected is adjusted. The provision of the longitudinal groove 41 in the plunger 8 is to keep fuel in a noninjected state. When the plunger 8 is rotated to allow the longitudinal groove 41 and the cut-off port 36 to be positioned within the same plane, they are open to each other before the lateral hole 42 in the plunger 8 is closed by the control sleeve 24, so that the fuel-pressurizing chamber 9 communicates with a fuel reservoir chamber (not illustrated) by way of the longitudinal hole 31, inclined groove 40 and longitudinal groove 41 in the plunger 8 as well as the cut-off port 36 in the control sleeve 24, thereby keeping the fuel in a noninjected state.
However, since the prior art arrangement as described above is of the structure that the inclined groove 40 in the plunger 8 is directly open to the longitudinal hole 31 extending therein, a relatively large diameter of the longitudinal hole 31 is required. This results in the relatively large volume of a portion of the fuel-pressurizing chamber 9 from which an amount of fuel escapes during pumping, i.e., the dead volume, and is responsible for a drop in the performance of the fuel injection pump such as a lowering of the pressure of fuel to be injected.